Method of electrolytically precipitating chromium metal from aqueous chromium (vi) oxide solutions



3,444,060 METHOD OF ELECTROLYTICALLY PRECIPITAT- ING CHROMIUM METAL FROMAQUEOUS CHROMIUM (VI) OE SOLUTIONS Hans Rothmann and Werner Keil,Nuremberg, and Heinz Richter, Furth, Bavaria, Germany, assignors toGesellschaft fur Elektrometallurgie m.b.H., Dusseldorf,

Germany No Drawing. Filed July 28, 1964, Ser. No. 385,777 Int. Cl. C22d1/24, N

US. Cl. 204-105 4 Claims ABSTRACT OF THE DISCLOSURE The presentinvention relates to a method of electrolytically precipitating chromiummetal from aqueous chromium (VI) oxide solutions.

As known, metallic chromium is electrodeposited on metal objects fromchromium (VI) oxide (CrO baths. Recently methods have been described ofelectrolytically recovering chromium metal from chromium (VI) oxidebaths for metallurgical purposes. Several different kinds of procedurecan be distinguished. For instance, according to one proposal known tothe art, a single addition of chromium (1H) sulphate and of chromium(III) oxide (Cr O in certain proportions is introduced into a chromium(VI) oxide electrolyte for the purpose of depositing thick layers ofchromium. According to another proposal sulphuric acid, hydrofluoricacid or fluosilicic acid is added to a chromium (VI) oxide electrolytefrom which varying yields of metallic chromium with varying contents ofoxygen are deposited. In the production of metallic chromium formetallurgical purposes from aqueous chromium (VI) oxide solutions thenature of other acids used as catalysts in the electrolyte is a matterof considerable importance. For example, if the chromium (VI) oxideelectrolyte contains sulphuric acid or sulphate ions, then a low currentyield of a sulphur-containing and, according to the conditions ofelectrolysis, particularly according to temperature, of anoxygen-containing chromium metal are obtained. However, if the chromicacid electrolyte is free from sulphate ions and the catalyst isexclusively fluosilicic or hydrofluoric acid, then a much better yieldof a very pure metal is deposited from such solutions than that obtainedfrom sulphate-ion-containing electrolytes. Hence, whereas in thetechnique of electrodeposition for forming metallic chromium coatingswhich are only a few microns thick the nature of the additional aciddoes not greatly matter, it does matter very considerably if a very purechromium metal is to be recovered for metallurgical purposes. Chromiumtrioxide solutions of high purity must be used. The market price ofchromium trioxide is high; it is also toxic and corrosive. Furthermore,from the method used for producing it, it still contains sulphate.Before it can be used as an electrolyte it must therefore be purifiedbecause the sulphate ions would otherwise accumulate and the electrolytewould be poisoned.

Purification of chromium trioxide by recrystallisation is difficult andexpensive owing to its solubility in water and its high corrosiveactivity. The precipitation of the sulphate ions with barium is likewisecomplicated and involves the risk of introducing barium into theelectrolyte.

It has now been found that substantially pure chromium can be producedfrom an aqueous solution of chromium trioxide and the above describeddifficulties overcome if, according to the invention, solid chromicoxide is added to the chromium trioxide solution. The method can becontinuously performed by adding the chromic oxide at 'the same rate aschromium metal is deposited at the cathode. Unlike chromium (VI) oxide,chromium (III) oxide is cheap, non-toxic and has no corrosive effect.The chromic oxide dissolves in the original chromium trioxide solutionand is oxidised to chromium trioxide at the anode.

It is advisable to use readily filtrable water-containing chromic oxideWhich is sufliciently reactive to be oxidised at the anode to chromiumtrioxide and is also sufficiently pure to permit continuous additionthereof, according to the invention, to function as a chromium supplierto the aqueous chromium trioxide electrolyte without poisoning theelectrolyte. The addition is made at the same rate as metallic chromiumis deposited at the cathode. Preferably, the chromic oxide is theproduct of precipitation from alkali chromate or alkali dichromatesolutions.

According to a further feature of the invention, the chromic oxide isthe precipitation product of alkali chromate and alkali dichromatesolutions containing Cr in concentrations between 20 and 200 grams perliter reacted with organic reducing compounds, preferably wood meal, for1 to 3 hours at temperatures between 200 and 300 C. and preferably atabout 250 C. The reaction may be carried out in an autoclave and forms areadily filtrable chromic oxide. Since commercial alkali chromate oralkali dichromate may contain up to 0.5% sulphur in the form of thesulphate, such a high sulphur content would lead to the production of achromic oxide containing between 0.02 to 0.04% of sulphur. This sul phurcontent is objectionable when the oxide is to be used as a chromiumsupplier in the electrolysis of the chromium trioxide electrolyte. It istherefore advisable to reduce the sulphur content by precipitating thesulphur with an equivalent proportion of barium salt-preferably bariumchlorideat a pH of 2 from the alkali chromate or dichromate solutionsprior to their reduction. This precipitation may be performed either atelevated temperatures or at room temperatures. The loss of chromium inthe precipitated barium sulphate is under 1%. An alkali-containingchromic oxide obtained from a desulphurated alkali dichromate solutionhas a residual sulphur content below 0.002%.

The sulphur-free chromic oxide thus obtained still contains 1 to 5% ofNa O, probably in the form of sodium chromite, after having beenfiltered and washed with water. This high alkali content would alsocause trouble when the oxide is used as a chromium supplier in theelectrolysis of chromium trioxide. According to the invention thisalkali content of the chromic oxide can be neutralised with or withoutthe application of heat with a chromium trioxide solution which is freefrom sulphate and Washed out in the form of sodium chromate ordichromate, the chromic oxide at the same time absorbing a quantity ofchromium trioxide roughly equivalent to the alkali oxide, a circumstancewhich substantially assists in promoting the subsequent oxidation of thechromic oxide to chromium trioxide in the electrolyte. A chromic oxidewhich has been thus treated has an alkali content below 0.1%. However,in performing the washing process care must be taken to ensure that thechromic acid contains no sulphate ions because if a chromic acidcontaining sulphate were used the alkali would of course be washed outof the chromic oxide and the extremely reactive trioxide absorbed, butat the same time sulphate ions would also be taken up from thesulphatecontaining solution of chromic acid. It is therefore a matter ofimportance to ensure that a chromium trioxide solution containing nosulphate is used for removing the alkali. The best course to take is touse the sulphatefree chromium (VI) oxide electrolyte.

The dilute alkali chromate or dichromate solutions obtained by washingout the alkali are reused for producing alkali chromate or dichromatesolutions, so that chromium losses cannot occur during the performanceof the method.

In order that the method proposed by the present invention can morereadily be understood a number of examples will be described in greaterdetail.

EXAMPLE I 5.74 kg. of crystallised, commercial sodium dichromatecontaining 0.3% of sulphur were dissolved in 20 litres of water. As soonas the salt had dissolved the pH of the solution containing 100 g. ofchromium per litre was adjusted to pH 2 with the aid of a pH meter.About 1.5 millilitres per litre of hydrochloric acid (1.19) were needed. The sulphate ions were precipitated by the addition of 110 g. ofcrystallised barium chloride, dissolved in 200 millilitres of water.When precipitation was performed at room temperature and whilst stirringthe solution, 99% of the sulphur was precipitated in the form of bariumsulphate in 192 hours. The chromium which was carried down by theprecipitate amounted to less than 1% of the total chromium introduced.Experiments have disclosed that the completeness of the precipitation inthe stated conditions depends upon time. For instance at the end of 60hours only 94% of the sulphur had been precipitated as barium sulphateand the chromium loss was The duration of the precipitation period couldbe reduced by heating the suspension. When the temperature was raised to80 99% of the sulphate were precipitated as barium sulphate in 24 hours,the chromium loss being under 1%. After precipitation of the sulphur theclear solution was drawn off and poured into an autoclave of 30 litrescapacity. 1 kg. of wood meal was added, the autoclave being sealed, thecontents kept for one hour at 250 whilst being stirred and then cooled.A 99% yield-related to the chromium introduced into the solutionof abrilliant green, readily filtrable granular hydrated chromic oxide wasobtained which was removed from the liquid on a suction filter. Theoxide was washed with pure water until all alkalinity of the filtratehad disappeared (quantity of wash water used being about 100 litres).Filtrate and wash water were discarded. After having been dried for 2hours at 105 the oxide had the following composition:

Percent (31 203 Na O 2.5-3.5 Fe O 0.008 S 0.002

Remainder water.

For removing the alkali the moist chromic oxide on the suction filterwas wetted with 3.3 litres of dilute sulphate-free chromic acidelectrolyte (80 g. of Cr per litre) by adding the latter in portions. Assoon as the entire washing solution had been absorbed by the chromicoxide the latter was rewashed with litres of pure water with theapplication of gentle suction. The filtrate thus obtained containedabout half the chromium trioxide from the wash solution, the remainderof the chromium trioxide solution having been retained in the chromicoxide.

Percent Cr O 71-75 Na O 0.05 F6203 0.00s s 0.002

Remainder Water.

EXAMPLE II Production of the chromium (VI) oxide electrolyte withchromium (III) oxide addition In order to provide the initial solution100 g. of pure chromium trioxide and 6 g. of fluosilicic acid weredissolved in 1 litre of pure water and poured into a lead vesselconnected to operate as an anode. A steel rod (dia. 1 cm.) served as thecathode. Whilst moist chromic oxide which had been prepared as describedin Example I was continuously added its oxidation was effected with acurrent of 20 amps at 4 volts. At an electrolysing temperature of C. thechromium (VI) ion concentration in the solution rose by 7 to 8 grammesper hour in the solution. The current yield at the anode was 60 to 80%.A slight metallic deposit formed at the cathode. Oxidation was continueduntil 10 litres of a chromium (VI) electrolyte having the desiredconcentration of 156 g./ litre of chromium had been obtained. 2 litresof this solution were used for removing the alkali from the chromicoxide. 50 g. of fiuosilicic acid were added to the remaining 8 litres,the cathode was replaced by a steel cathode (dia. 6 cm.) andelectrolysis was performed with a current density at the cathode of 100amps/cm. at a voltage of 7 volts at C.

During the process 80 g. (=28 g. of Cr) of chromic oxide, moist from thesuction filter and free from sulphur and alkali, were added per hour.The total quantity of chromium added in the course of 29 hours was 800grammes. 840 grammes of chromium metal were recovered. When electrolysiswas stopped the electrolyte contained 151 g./litre of chromium in theform of chromium trioxide. The added chromic oxide had been fullydissolved and oxidised to the trioxide during electrolysis. The chromiummetal recovered in this way contained 0.04% of O and -60 millilitres ofH g. The sulphur content was less than 0.01% and the total of othermetallic impurities amounted to less than 0.0005

We claim:

1. A method of continuously producing pure chromium by electrolyticdeposition from an aqueous solution of chromium (VI) oxide in thepresence of fluosilicic acid or hydrofluoric acid, comprising the stepof adding to the solution a solid pure activated hydrated chromium(II-I) oxide at a rate substantially corresponding to the rate ofdeposition of metallic chromium on the cathode and recovering thedeposited chromium from the cathode, the said solid pure activatedhydrated chromium (III) oxide being obtained by reducing a solution ofsubstantially sulphur-free alkali chromate or alkali dichromate with anorganic reducing agent at an elevated temperature in an autoclave, andfiltering off the resulting chromium (III) oxide.

2. In a method of continuously precipitating and recovering chromium formetallurgical purposes by electrolytic deposition of the chromium on acathode from an aqueous solution of chromium (VI) oxide in the presenceof fluosilicic acid or hydrofluoric acid, the improvement comprisingadding to the solution solid substantially pure readily filtrableactivated, hydrated chromium (III) oxide, which is substantiallysulphur-free, at a rate substantially corresponding to the rate ofdeposition of metallic chromium on the cathode and recovering thedeposited chromium from the cathode, whereby the chromium (III)References Cited oxide dissolves in the solution of chromium (VI) oxideUNITED STATES PATENTS and is oxidized at the anode to form chromium (VI)oxide and functions as a chromium supplier to the aqueous 662 5 5solution. 5

3. The process of claim 2 wherein the said chromium 3259560 7/1966Brandes et 204-105 (III) oxide is the precipitation product from alkalichro- FOREIGN PATENTS mate and dichromate solutions which chromates have829,429 3/1960 Great Britain.

been reduced.

4. The process of claim 2 wherein the chromium (III) 10 HOWARD WILLIAMSPr'mary Exammeroxide is substantially alkali-free. H. M. FLOURNOY,Assistant Examiner.

